C-arm systems are frequently employed in medical practice. In the case of these systems, the x-ray detector and the x-ray source are affixed opposite each other on a so-called C-arm, which is embodied for the execution of a rotational movement. The image recording system can thus rotate with the x-ray source and the x-ray detector about a center of rotation, the so-called isocenter of the C-arm system. In this way, these modern C-arm systems enable the creation not just of two-dimensional fluoroscopic images, but also, through rotation of the image recording system around the patient, three-dimensional, CT-like images or cross-sectional images. The image reconstruction of the three-dimensional images or the cross-sectional images from the fluoroscopic images recorded subject to different angles of rotation takes place in a similar manner to those produced with a computer tomograph (CT). Soft tissue or, through the subtraction of contrast agent images and native images, angiographs can be represented in three-dimensional form.
The fluoroscoped object area, from which through reconstruction, a three-dimensional image can be gained, is limited by the size of the beam cone of the X-ray radiation beam. The size of the beam cone is in turn determined by the size and the distance of the x-ray detector, as a rule a flat panel detector. The area of interest of an object, known as the VOI (Volume of Interest), must lie in the isocenter of the C-arm system, so that it can be mapped by the image recording system for every angle of rotation or projection. In the case of areas of interest of objects in medical imaging which are centrally located within the cross-section of the patient, for example the head, the heart or the central abdominal area, this condition is fulfilled as a matter of course. If, however, areas at the periphery of the patient cross-section, such as the liver or the kidneys, are to be represented, problems arise on account of the above-mentioned condition. These areas can only be brought into the isocenter of the C-arm system to a limited degree, as lateral relocating of the patient or the patient support table is severely restricted by the rotating C-arm and its attached image recording system.
This is illustrated in diagrammatic form in FIG. 1, which shows a C-arm 1 with x-ray tube 2 and x-ray detector 3 in near-lateral projection during rotation around the stylized patient 4, who is positioned on the patient support table 5. A three-dimensional image of an area 6 lying within the isocenter here presents no problem. If, however, areas at the periphery of the patient cross-section are to be imaged, for example the area of interest 7 indicated in FIG. 1, the patient is currently laid on their side on the table, or the table relocated to one side (to the right in FIG. 1). The lateral relocation is, however, severely restricted because of the danger of collision with the x-ray tubes rotating about the isocenter or the rotating x-ray detector, and is thus frequently not possible to the extent necessary for the three-dimensional imaging.